During a SAG mill operation as with other components within thereof, the grates experience wearing due to the movement of the internal load which generates abrasive actions on the liners as well as direct impacts from the elements used for the grinding operation, i.e., the steel balls. On the other hand, during the operation of the mill and the variations in the content of the internal loading, due to the effect of the granulometry of the ore fed or ore hardness, variations are generated in the composition of the internal load. If these variations mean a decrease in the ratio of ore inside thereof, there is a greater amount of mill balls inside thereof, thus explaining the mechanical demand on the components, particularly grates as they are subject to an excess of impacts from the elements designed to hit the ore. This condition might result in the impacts between balls and the grate produce an even greater wearing than the usual or that they reach a level of structural resistance thus producing fracture of such elements, especially during the last stage of their working life.
Fracture in a discharge grate forces stoppage of the mill for periods ranging from four to twelve hours (or more), depending on the size of the mill and the degree of damage produced to the components, which has the impact of significant production losses.
Controlling the conditions of such components in mill liners is performed by stopping the mill as to carry out the inspection, which generates an important loss in production. During such stoppages the condition of the elements are visually checked and the thickness of the grates ribs is measured as to determine the remaining useful life.
During said inspections, it is possible to find situations of clogging in the discharge grates as the balls are embedded in between the grate slots preventing the ore ground to the size of the slot to be discharged via the normal ways in the process. This condition is reflected in the decrease in the treatment capacity of the mill due to the increase in the retention time of the ore thereof, which translates in important production losses.
A number of attempts had been made in the state of the art aiming to provide real-time monitoring of the conditions under which the milling is being done while the mill is under operation. For example, in U.S. Pat. No. 6,874,364 (Campbell et al.) published on Apr. 5, 2005, discloses a system to monitor mechanical waves in a machine that has particles in motion when in operation, wherein the system includes at least a sensor located in the machine at a distal location from the central axis of the machine, and the sensors are designed to detect acoustic waves and include a transmitter to transmit signals representing the mechanical waves detected to a receiver located in a remote location from the sensor(s), a data processor connected to the receiver to receive signal from the receiver representing the mechanical waves and to process signals as to produce output signals for further visualization in a screen, where the output signals represent one or more parameters indicative of the mechanical waves produced by the machine during a specific period of time.
Document U.S. Pat. No. 5,698,797 (Fontanille et al.) published on Dec. 16, 1997, discloses a monitoring device for a ball mill which has a group of balls arranged, during the rotation of the mill at a normal speed, between two generators (Ib, Ib) separated to a minimum angle (α) and a maximum angle and a mass of coal arranged during the rotation of the mill at a normal speed between two generator (Ic, Ic) separated in an angle (β), and which consists of a wave transmitter, waves selected from between the electromagnetic waves, wherein said transmitter can be arranged within the mill, and receiver means for such waves, wherein said receiver means are connected to an electronic circuit to determine the parameters corresponding to the number of balls, the amount of coal, and the wearing of the cover, where such means can be arranged in the external part of the mill in such a way that they can detect the waves crossing a generator Ib and the waves in the external part of the maximum angle sections and β, as to determine the wearing of the cover; and that they can detect the waves in the angle section β not common to the angle section in order to determine the amount of coal. The wave receiver means are arranged in a rotational manner around the longitudinal axis of the cover in an angle section above the angle section encompassing α and β. 3. In this system the transmitter is located in the longitudinal axis of the cover whereas such transmitter is a gamma-ray photon type transmitter. The electronic circuit to determine the number of balls include, for each generator (Ib, Ib), one converter and one lineariser, wherein the signals from each lineariser are associated as to calculate the number of balls. The electronic circuit to determine the wearing in the case consists of a converter connected to a device to read the degree of wearing.
Document DE 4215455 (Godler) published Nov. 18, 1993, discloses a system with sensors for sound signals produced as a response to the noise generated by the milling plant, signals that are then analyzed as to render a measurement value of the status of operation of the plant. The status of the operation is measured as per the level of the mill. In order to analyze the noise, the system creates a noise spectrum and includes a device for a fast Fourier transformation. It also includes a device that creates the average of the spectrum during a long period of time. This system allows determining the performance of the mill, particularly a mill for rocks, in order to improve it and to improve the quality of the processed material.
The three documents described above disclose methods and apparatus that detect noise and make the correlation of said noise as to determine some of the operation properties. However, none of said documents teach how to detect the thickness in the ribs of a SAG mill discharge grates, and not even how to place them within the mill thereof.